Recent data show that brown adipose tissue (BAT) plays an important role in the regulation of energy balance in rodents. We have shown that BAT contributes to the thermic effect of single meals (specific dynamic effect - SDE). Though the existence of SDE has been known for about two centuries, its origins have not been hitherto clearly identified. In this proposed study, the contribution of BAT to the overall thermic effect of single meals will be quantified and the effects of the macronutrient composition of the test meal on the magnitude of SDE originating from the BAT determined. Metabolic parameters involved in the mechanism of this heat production and the effect of the plane of nutrition on it will be determined. Dietary induced thermogenesis will be determined during chronic overfeeding as it is affected by the macronutrient composition of the diet. The relationship between the heat production for BAT in response to single meals and in response to chronic overfeeding will be characterized from the effects of macronutrient composition on the two phenomena, from the metabolic parameter involved in the two processes and from the effect of the plane of nutrition on the BAT thermic response to single meals. A hypothesis will be tested that dietary obesity that develops on a high fat or a high sucrose diet results from a reduced BAT thermic response to single meals upon consumption of these nutrients. A second hypothesis will be tested, according to which heat production from BAT serves as a feedback signal for satiety. Preliminary data in our laboratory tend to support the two hypotheses.